A magnetic recording medium, e.g., a hard disk, typically comprises a laminate of several layers, including a non-magnetic substrate, such as of aluminum-magnesium (Al—Mg) alloy or a glass, ceramic, or glass-ceramic composite material and formed sequentially on each side thereof: a polycrystalline underlayer, typically of chromium (Cr) or Cr-based alloy, a polycrystalline magnetic recording medium layer, e.g., of a cobalt (Co)-based alloy, a hard abrasion-resistant, protective overcoat layer, typically carbon (C)-based, and a lubricant topcoat layer. Magneto-optical (MO) media, e.g., in disk form, similarly comprise a laminate of several layers, including reflective, dielectric, thermo-magnetic, protective overcoat, and lubricant topcoat layers.
In operation of e.g., the magnetic recording medium, the polycrystalline magnetic recording medium layer is locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based upon the bits of information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, then the grains of the polycrystalline recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of magnetization matches the direction of the applied magnetic field. The magnetization of the polycrystalline recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
Thin film magnetic and MO recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (CSS) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance above the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, thereby allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
As a consequence of the above-described cyclic CSS-type operation, the surface of the disk or medium surface wears off due to the sliding contact if it has insufficient abrasion resistance or lubrication quality, resulting in breakage or damage if the medium wears off to a great extent, whereby operation of the disk drive for performing reading and reproducing operations becomes impossible. The protective overcoat layer is formed on the surface of the polycrystalline magnetic recording medium layer so as to protect the latter from friction and like effects due to the above-described sliding action of the magnetic head. Abrasion-resistant, carbon (C)-containing protective coatings have been utilized for this purpose, and are typically formed by sputtering of a carbon target in an argon (Ar) atmosphere. Such amorphous carbon (a-C)-containing protective overcoat layers formed by sputtering have relatively strong graphite-type bonding, and therefore exhibit a low coefficient of friction in atmospheres containing water (H2O) vapor, which characteristic is peculiar to graphite. However, the a-C layers produced in such manner have very low hardness as compared with many ceramic materials such as are employed as slider materials of thin film heads, and thus are likely to suffer from wear due to contact therewith.
In recent years, therefore, carbon-based protective overcoat layers having diamond-like hardness properties (i.e., HV of about 1,000-5,000 kg/mm2) have been developed, and films of diamond-like carbon (DLC) having a high percentage of diamond-type C—C bonding have been utilized. Such DLC films exhibit a high degree of hardness due to their diamond-like sp3 bonding structure, and in addition, exhibit the excellent sliding properties characteristic of carbon, thus affording improved sliding resistance against sliders composed of high hardness materials. Such DLC films are generally obtained by DC or RF magnetron sputtering of a carbon target in a gas atmosphere comprising a mixture of Ar gas and a hydrocarbon gas, e.g., methane (CH4) or hydrogen (H2) gas. The thus-obtained films exhibit DLC properties when a fixed amount of hydrogen is incorporated therein. Incorporation of excessive amounts of hydrogen in the films leads to gradual softening, and thus the hydrogen content of the films must be carefully regulated.
Amorphous, hydrogen-doped, i.e., hydrogenated carbon (a-C:H) films obtained by sputtering of carbon targets in an Ar+H2 gas mixture and exhibiting diamond-like properties have also been developed for improving the tribological performance of disk drives; however, the electrical insulating properties of such type films lead to undesirable electrical charge build-up or accumulation over time during hard disk operation which can result in contamination, glide noise, etc. In order to solve this problem without sacrifice or diminution of the advantageous mechanical properties of such a-C:H films, attempts have been made to dope or otherwise incorporate nitrogen (N) atoms into the a-C:H films, in view of a substantial decrease in electrical resistivity and optical band gap (EBG) exhibited by such N-doped a-C:H films relative to undoped films. In addition to these hydrogen-containing DLC materials, amorphous as well as crystalline DLC films and coatings comprising compounds of carbon and nitrogen (CNx) have also been developed and evaluated for use as protective overcoat layers for magnetic recording media.
More recently, protective overcoats comprised of multiple layers of differently constituted carbon-based materials have been developed with the aim of providing magnetic recording media with optimal tribological (e.g., hardness) properties while at the same time providing one or more of good adhesion, low surface energy, reduced stiction, and superior corrosion protection. For example, U.S. Pat. Nos. 5,607,783 and 5,942,317 disclose formation of protective overcoats comprising multiple layers (e.g., 2-3) of C:H, each of the multiple layers having a different amount of hydrogen atoms incorporated therein. Typically, the content of hydrogen is highest in the uppermost layer and least in the lowermost layer. U.S. Pat. Nos. 5,714,044; 5,785,825; and 5,855,746 disclose formation of carbon-based multilayer protective overcoats wherein the lowermost layer in contact with the recording media layer surface comprises undoped carbon and the uppermost, exposed layer comprises carbon (C) doped with nitrogen (N) or hydrogen (H).
The above-mentioned undoped, amorphous carbon-based protective overcoat layers (a-C) are generally formed by sputtering of a carbon (C) target in an atmosphere comprising a rare gas, typically argon (Ar). Hydrogen-doped and nitrogen-doped amorphous carbon (a-C:H and a-C:N, respectively) films can be similarly produced by sputtering; however, the argon (Ar) gas is mixed with a certain amount of hydrogen (H2) gas or nitrogen (N2) gas. Although the doped amorphous carbon films are not as hard as undoped amorphous carbon films, they exhibit a significantly higher resistance to tribochemical wear than the undoped films. The overall performance of doped amorphous carbon films is thus superior to that of undoped amorphous carbon films, resulting in their currently being extensively employed in the manufacture of magnetic recording media.
However, a common problem encountered in the formation of doped amorphous carbon films utilized as protective overcoats for magnetic and/or MO recording media is arcing (i.e., sparking) from the carbon sputtering target which can damage or result in defects in the disk and its laminate of layers including the magnetic recording layer. Such damage is commonly termed carbon-induced damage (CID), which damage can adversely affect the information storage properties of the disk. Such arcing is believed to originate from the development of positive charge at local irregularities or foreign particles on the carbon target. Because the target is negatively biased during sputtering, a voltage is developed between the target and the local positive charge(s0 on the carbon target. When the voltage becomes sufficiently high, a dielectric breakdown can occur which causes a large electric current density (i.e., arcing) in the local area. The arcing can, in turn, result in sputtering of glassy nodules present on the target surface, which sputtered glassy nodules can deposit on the disk surface and result in CID.
In addition to the above-described difficulty associated with sputtering of carbon targets, an additional factor providing impetus for the development of non-sputtering techniques for depositing carbon-based protective overcoats arises from the continuous increase in areal recording density of magnetic recording media which, in turn, requires a commensurately lower flying height of the transducer head. Therefore, it is considered advantageous to reduce the thickness of the carbon-based protective overcoat layer (or multilayer) without incurring adverse consequences. Conventional sputtered a-C:H and a-C:N materials are difficult to uniformly deposit in defect-free manner for the reason given above, and generally do not function satisfactorily in hard disk applications at reduced thicknesses. Therefore, the use of alternative deposition techniques for developing thinner and harder DLC layers having the requisite mechanical and tribological properties has been examined, such as, for example, chemical vapor deposition (CVD), ion beam deposition (IBD), and cathodic arc deposition (CAD) techniques. Of these, the IBD method has demonstrated ability to be utilized for forming undoped and doped ion beam-deposited carbon films that exhibit superior tribological performance at reduced thicknesses.
Conventional circularly-configured wide ion beam sources typically utilized for the deposition of IBD carbon-based films or coatings, such as Kaufman, and gridless end-Hall and enclosed-drift end-Hall sources, are described in Handbook of Ion Beam Processing Technology, J. J. Cuomo et al., editors, Noyes Publications, Park Ridge, N.J., pp. 40-54, and in U.S. Pat. Nos. 4,862,032; 5,192,523; 5,482,602; and 5,508,368, the entire disclosures of which are incorporated herein by reference. Such type ion beam sources typically operate at pressures below about 1 mTorr in order to minimize the collision of energetic ions forming the ion beam with ambient energy molecules of the background gas, enable formation of an intense, highly ionized plasma, and thus permit carbon films to be obtained which exhibit optimum properties, e.g., hardness, for use as protective overcoat materials in hard disk applications. DLC materials in film or coating form can be produced on suitable hard disk substrates located in the path of the ion beam produced by such ion beam sources by introducing a hydrocarbon source gas (additionally with admixed nitrogen gas, if desired) into the ion beam exiting the orifice of the source or by passing the source gas(es) through the ion beam source from the rear thereof.
The ion beam source is typically integrated with sputtering equipment for continuous, automated manufacture of hard disks. More specifically, a typical automated hard disk manufacturing system includes at least one linearly elongated or circularly-shaped main vacuum chamber having a number of process stations serially arranged therein, each dedicated for deposition of a distinct material layer on the hard disk substrate or to another type of treatment, e.g., etching, cleaning, etc. When such systems are employed for the manufacture of magnetic or MO recording media, e.g., hard disks, each process station typically comprises a sub-chamber maintained under high vacuum conditions, e.g., for sequentially depositing on the hard disk substrate, as by cathode sputtering or other suitable technique, e.g., IBD, a respective one of the various layers comprising the recording medium. Workpiece (i.e., substrate) handling/transfer means are provided for transferring the substrates, in sequence, from a preceding station to a following station, with substantially distinct atmospheric conditions being maintained within each sub-chamber, depending upon the particular processing performed therein.
However, when depositing DLC layers for wear protection, whether by means of sputtering or IBD, it is often advantageous for the reasons given above, to deposit two or more differently constituted layers of carbon-based protective overcoat materials, each expressly grown to provide different film properties (e.g., hardness, wear resistance, adhesion, corrosion protection, stiction, etc.). Particularly in the case of IBD, it is often not possible for space reasons or practical for cost reasons to utilize more than one (e.g., 2-3) IBD sources in the automated hard disk manufacturing apparatus as described supra, inasmuch as the process chambers and associated vacuum pumps employed with IBD apparatus are quite large relative to typical sputter deposition sources, process chambers, and vacuum pumps.
Accordingly, there exists a need for an improved method and apparatus for forming multilayer films or coatings by IBD, particularly, but not limited to, multilayer carbon-based films suitable for use a protective overcoats for magnetic and/or magneto-optical (MO) recording/information storage and retrieval media which method and apparatus overcome the above-described drawbacks and disadvantages associated with conventional IBD technology, yet are simple, cost-effective, and fully compatible with the productivity and throughput requirements of automated disk manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the formation of multilayer films or coatings, e.g., protective overcoats suitable for use with high recording density magnetic and/or MO recording media, while maintaining full compatibility with all manufacturing and performance aspects of conventional disk drive technology.